Introduction
In dairy cow herds, we always aim for the calf to have a suitable weight at birth and safely pass the lactation period, so that female calves can be used as dairy cows with proper performance in the herd. Nowadays, most dairy cattle farms have a relatively high genetic capacity, which necessitates adherence to management points of calf rearing such as consumption of solid and liquid feed, the importance and manner of rumen development and its effect on calf performance, starter consumption, and health and performance after weaning. Following these management practices on farms can help utilize this capacity to improve herd performance.
In order to enter the breeding program of the herd (Holstein breed up to the age of approximately 13 months) in the suckling calves, it is important to observe different weighing standards for different breeds. From the time they stop consuming milk until they start consuming growth feed (TMR), in order to utilize their maximum genetic capacity and improve the performance of the herd. In general, a birth weight of 6% of maturity weight is expected, at weaning about 12% of maturity weight, before entering the breeding program 45% of maturity weight, and at the time of entering the artificial insemination program 55% of maturity weight. The animal is expected to reach 85% of maturity weight in the first birth, 90% in the second birth, and finally, physical maturity in the third birth.
The importance of calf growth and performance in dairy herds
The importance of calf growth and performance in dairy herds is evident from numerous studies conducted worldwide, which have shown a direct relationship between daily weight gain in the first 6 weeks and milk production during the initial lactation period (first pregnancy). Research indicates that calves achieving a daily weight gain exceeding 500 grams (for every additional 100 grams beyond 500 grams) during the first calving period produced 113 to 155 grams more milk. Therefore, to enhance calf performance, it is desirable to aim for a minimum daily weight gain of 500 grams, with higher gains beyond this threshold correlating with increased milk production.
The question raised here is: How is weight gain achieved?
Is it through an increase in the DMI of consumed milk or an increase in the consumption of solid feed (starter)?
Calves that consume more milk are expected to have a higher weaning weight and average weight gain. However, it is important to consider that age and weaning weight have associated costs and consequences that make weight gain essential. Continuous weight gain post-weaning is crucial to prevent growth stunting in calves. In a study conducted in 2016, scientists found that for every 100g increase in dry matter intake above milk or liquid feed, there was an approximate increase of 66.2 kg in milk production during the first lactation period. Conversely, for every 100g increase in dry matter consumption above milk when used concurrently with starter feed, there was an estimated increase of 138.5 kg in milk production during the first lactation period. Therefore, the increase in weight resulting from solid food consumption can have a more significant impact on milk production in the first pregnancy. Weaned calves, despite having lower initial costs compared to dairy cows, are a significant part of the herd (representing approximately 50% of the total herd). If this percentage falls below 50%, and our productive percentage is higher in the herd, we can reduce capital expenses and cover farm costs through milk sales. Breeding objectives and economic values should be considered in herd management decisions.
Breeding goals and economic values
As livestock age, feed yield decreases, leading to a normal decrease in feed costs. It is important to note that after weaning, the cost of feed per kilogram of body weight decreases until the animal becomes pregnant. Subsequently, feed costs will increase post-pregnancy. Studies indicate that achieving better performance from the calf after weaning until insemination resulting in pregnancy offers several advantages: 1- Lower cost for weight gain. 2- This type of growth is protein-based, and until the animal becomes pregnant and progesterone hormone secretion increases, it naturally stores less fat in the body and the breast’s parenchymal tissue grows more. This period, known as the period of opportunity, from weaning to insemination leading to pregnancy, should be maximized. If the calf successfully transitions through weaning and consumes sufficient solid feed, it will naturally avoid growth stunting post-weaning.
Increasing the consumption of milk is not a good solution to achieve a higher ADG!
In previous studies, it was reported that calves that consumed more milk gained more weight until 7 weeks of age, while the group that consumed less milk showed less weight gain. However, upon entering the weaning program, it was observed that the group that consumed more milk lost more weight, causing the growth curve of the calves to shift from a linear state to a sinusoidal state. This weight loss was sometimes not compensated for until 12 weeks after birth.
Pre-weaning performance has an effect on the post-weaning growth curve.
Ruminal health and development in weanling calves are crucial for success, as they help prevent the animal from deviating from linear growth after weaning. If the calf is healthy but the rumen development is inadequate, it may result in a plateaued and wrinkled growth curve. These calves may experience increased stress when transitioning off milk, indicating that their digestive system is not yet prepared to consume solid food.
Ruminal development is the most important issue after colostrum consumption in a nursing calf.
Physical development (consumption of solid food, fibers)
Metabolic development or growth of epithelial tissue (volatile fatty acids)
Microbial development (consumption of solid feed and water)
In the digestive system of dairy cows, the rumen increases its share with age. By the age of 12 weeks, it covers approximately 65% of the volume of the stomach and pre-stomach. During the first week, milk is transferred to the calf’s teat through the gullet, where it coagulates, digests, and ultimately enters the intestine. As a result, the share of milk decreases from 60% during lactation to 20% in adulthood. An adult animal allocates about 80% of the volume of the digestive system in the stomach and pre-stomach to the rumen.
In metabolic development, what needs to be checked are not the volume of the digestive system, but the changes in the rumen epithelium tissue that are important. At birth, the rumen epithelium tissue does not change much, but with increasing age (3 to 4 weeks), the rumen epithelium tissue grows and develops. The consumption of dry matter increases the length of the villi and helps to increase the level of absorption of nutrients. In one study, the inner surface of the rumen of calves fed only milk or milk with forage was completely smooth with small villi even at 12 weeks of age. However, the calves that consumed milk and starter at the same time developed rumen villi (increase in length and absorption surface) and the rumen color was darker. In fact, the larger the size of the bristles, the better the absorption level and therefore the performance of the calf.
What is important in rumen metabolic development is the role of volatile fatty acids.
Consumption of starter under the influence of several factors affects the metabolic development of the rumen:
When the consumption of dry matter increases in calves, ruminal fermentation occurs, leading to an increase in the production of volatile fatty acids (propionate and butyrate), especially in feeds with a high amount of concentrate. The amount of carbohydrate-decomposing bacteria (starch and sugars) also increases, resulting in a decrease in rumen pH.
1- The mechanism of action of propionate:
Propionic acid serves as the primary precursor of glucose in ruminants. The rise in propionate production post-absorption into the bloodstream triggers an increase in insulin concentration in the blood. Insulin plays a crucial role in cell proliferation and stimulation, as well as epithelium growth. Consequently, blood circulation around the rumen escalates, leading to the growth of rumen villi.
When the primary fermentation in the rumen consists of butyric and propionic processes, they significantly impact rumen development. Acetic acid, primarily produced by cellulolytic bacteria, has a lesser influence in this aspect.
2- Butyrate’s mechanism of action:
Feeds that are based on seeds or grains will have a greater impact on rumen development compared to forage feeds. This is due to the higher energy content in feeds such as starter, which results in increased production of propionic acid and butyric acid. Butyric acid plays a crucial role in rumen metabolic development, with 85% of it being absorbed in the rumen, leading to increased growth. The majority of butyric acid is metabolized to acetoacetate and beta-hydroxybutyrate (BHBA) in the rumen epithelium. Oxidation in the rumen wall generates ATP in the presence of oxygen, causing blood flow to redirect towards the epithelium.
Poor rumen development:
Weight loss and weakening of the calf’s immune system are consequences of poor rumen development, which can easily make the calf susceptible to various diseases, stress after weaning, and stress near birth.
The role of solid feed in stabilizing rumen anaerobic microbes
The commencement of solid food consumption significantly influences the development of the microbial population in the digestive tract. Prior to consuming solid feed, the rumen environment contains a high concentration of aerobic bacteria. However, upon starting solid feed intake, the population of aerobic bacteria in the rumen environment will decrease within the first 2 or 3 weeks. Subsequently, anaerobic bacteria conducive to the rumen environment will begin to proliferate, leading to a decrease in rumen pH as feed consumption increases.
The definition of acidosis in weanling calves differs from adult ruminants. There is no need for excessive concern regarding the occurrence of acidosis in calves, as a rumen pH of up to 6.5 may be observed after the initiation of solid feed consumption, indicating an increase in the absorption of volatile fatty acids in the rumen, which is less detrimental to the calf’s health. Typically, in adult ruminants, a pH lower than 5.8 is considered subacute acidosis depending on the measurement method. However, in suckling calves, pH levels as low as 5 and 2.5 can be seen. Symptoms of acidosis in young calves include lethargy, anorexia, digestive discomfort, and in some cases, diarrhea and reduced rumen activity. Calves with well-developed rumens exhibit efficient absorption of volatile fatty acids and can navigate acidosis crises more easily than others, returning to normal pH levels more quickly.
If the consumption of solid feed (starter) is low at the time of weaning!!
n general, post-weaned calves tend to have loose stools, which is attributed to the stress of weaning and elevated cortisol levels, ultimately resulting in diarrhea. Larger weaned calves will grow in size and significantly boost their intake of solid feed after weaning. Consequently, the rumen will produce high levels of volatile fatty acids, and a calf without proper metabolic development in the rumen will struggle to absorb these acids. This can lead to decreased feed consumption and growth deficiencies in the affected animals.
The relationship between starter intake and daily weight gain in the week leading up to weaning
The higher the feed intake, the more weight calves will gain (more than 500 grams per day) at the time of weaning. It was also reported that calves consuming less milk substitute showed higher dry matter consumption.
Consumption of starter increases the contribution of MCN to the total nitrogen entering the intestine.
Another advantage of using solid feed or starter in suckling calves, which is less noticed, is the role of microbial protein in the rumen in providing the protein needed by the animal. Microbial protein has the best profile of amino acids and the highest biological value, and studies have shown that the calves that consumed up to 1.5 kg of starter accounted for nearly 60% of the nitrogen entering the intestine, microbial nitrogen, which plays an effective role in providing protein. Metabolism in calves and the production of microbial protein, in addition to its advantages, is very cheap and low-cost.
